1. Field of Invention
The present invention relates to the field of linear measurement, specifically to optical methods of measuring nanometric distances between objects, such as a transducing head and a magnetic storage disk, that participate in relative motion.
2. Prior Art
At the present time, several optical techniques are used to measure a nanometric (microscopic) spacing between objects, e.g., between a magnetic head and disk of a computer disk drive. The smaller is the aforementioned spacing, the more accurate is the transduction of information stored on the recording media, i.e., the computer disk.
One measuring method is based on the phenomenon of interference between light beams, which is called optical interferometry. Interference is the mutual effect on meeting of two wave trains of light of the same type so that such wave trains produce lines, bands, or fringes either alternately light and dark or variously colored. When measuring separation between two objects having nearly parallel mutually facing surfaces, where one of the objects is transparent, a beam of light is directed into the gap to be measured through the body of the transparent object in such a way that the axis of the beam is essentially normal to the facing surfaces. Beams, reflected from the surfaces, are ultimately recombined at a detector element. The optical system is designed so that the path difference between the beams is related to the spacing the instrument is intended to measure. It is known from optics that the fraction of the original radiation collected by the detector depends in part on the ratio of the path difference to the radiation wavelength. This relationship is used as a calibration table for spacing measurements.
A particular application of optical interferometry, disclosed in U.S. Pat. No. 4,813,782 to Yagi et al., 1989, is the measurement of the nanometric space between a magnetic head of a computer hard disk drive and a flat reference disk. To simulate the working conditions of a hard disk drive, the reference disk is rotated at a high speed and the head, pressed downward by a spring, floats above the disk on a dense air cushion, which is created by the rotation of the disk. The head thus "flies" or has a "flight elevation" above the reference disk, so that the reference disk is used to dynamically test the flying behavior of the magnetic head. The reference disk is made of an optically transparent material, such as glass, and the light beam is directed through the disk from the side opposite to the magnetic head. Components of the beam, reflected from surfaces bounding the air cushion, eventually produce interference.
The main drawback of the above method is the inaccuracy of the calibration table, illustrated in FIG. 1, near its minimum and maximum points, where the flat regions in the curve significantly degrade measurement precision. Specifically, this is the case when the separation between the head and the disk approaches one quarter of the optical wavelength. Moreover, commercially available devices are unable to take measurements at several points on the magnetic head at a time. Therefore, time consuming point-by-point measurements have to be performed in order to obtain a map of surface-to-surface proximity. Currently, 90 nanometers is the smallest space that can be reliably measured using a commercially available spectrophotometric instrument to measure the spectral intensity distribution of the reflected light.
Another optical method that is used to measure the space between objects is known as frustrated total internal reflection. Total internal reflection may be observed when a light beam falls onto an interface between two media at an oblique incidence angle. If the light originates from the side of the denser of the two media and the incidence angle exceeds a certain critical value, all radiation energy is reflected back into the medium in which it originated.
It is further known that if the second medium is present in the form of a thin film and is followed by a third medium, which is more dense than the second, a part of the incident radiation can penetrate through the film and propagate into the third medium. The latter phenomenon is known as frustrated total internal reflection. In this case the fraction of radiation reflected back into the first medium is determined in part by the ratio of the thickness of the second medium to the radiation wavelength, in part by the complex refractive index of the third medium, and also by the polarization of incident radiation.
An apparatus which determines the proximity of a stationary glass surface to another surface by employing the phenomenon of frustration of total internal reflection of light energy from the glass surface is disclosed in U.S. Pat. No. 4,681,451 to John M. Guerra and William T. Plummer, 1987. The device, shown schematically in FIG. 2, is used to determine the gap between a magnetic head and a magnetic recording medium. In the mechanism, a glass block 110 is substituted for the conventional magnetic head, and a medium 120, e.g., a magnetic disk, may be set into motion to develop aerodynamic characteristics incident to establishing the spacing between a surface 118 and a glass surface 112. An optical fiber bundle 130 directs collimated light from a source 132 into block 110 through a planar surface 114. The pattern of light presented at a surface 116 as a result of total internal reflection from surface 112, due to proximity of surface 118 with surface 112, is magnified by a microscope 138. The enlarged image produced by microscope 138 is converted to a facsimile in which gray scale densities at coordinate locations throughout the area of the magnified image are recorded by a black-and-white television camera 140. The electronic facsimile of the image is fed into a three-dimensional oscilloscope 142. The image (FIG. 3) generated by the oscilloscope is directly convertible to a measurement of surface proximity.
The main disadvantage of this proximity imaging device is its inability to test the dynamic behavior and to measure the flight elevation of an actual magnetic head, as may be needed by a magnetic head manufacturer or consumer for quality control purposes. Even though the conditions inside a hard disk drive can be simulated by executing a replica of the head in glass, the results obtained in this manner are theoretical since physical properties, e.g., weight, of the glass head are different from those of an actual head. Furthermore, fiber-optic attachments, secured to the glass head, change its aerodynamic properties. Thus, the apparatus can not be used to test the characteristics of an actual head.